Reflector, method for fabricating the same and reflective liquid crystal display device incorporating the same

ABSTRACT

A reflector includes a substrate, a plurality of convex/concave portions formed on the substrate, and a thin reflective film formed over the convex/concave portions. When light is incident upon the reflector from a certain direction, an intensity of reflected light in a viewing angle range of about −45° to +45° with respect to a regular reflection direction of the incident light 10 is about 60% or more of an intensity of light which is incident upon a standard white plate from a direction inclined by about 30° from a direction normal to the plate and is reflected to the direction normal to the plate.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority from, and is a divisional applicationof U.S. patent application Ser. No. 08/806,438, which was filed on Feb.26, 1997, and which issued as U.S. Pat. No. 5,936,688 on Aug. 10, 1999.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a reflector incorporated in areflective liquid crystal display device with no backlight, a method forfabricating the same, and a reflective liquid crystal display deviceincorporating the same.

2. Description of the Related Art

In recent years, liquid crystal display devices have been increasinglyused in personal computers, TVs, word processors, video cameras, etc.There has been demands for further improvements of these appliances suchas miniaturization, power-saving, cost reduction, etc. In an attempt tomeet these demands, reflective liquid crystal display devices having nobacklight which display images by reflecting ambient light incidentthereupon have been developed.

In order to achieve bright display with a reflective liquid crystaldisplay device having no backlight, it is important to efficientlyutilize ambient light. Accordingly, a reflector incorporated in such areflective liquid crystal display device plays a very important roll. Itis thus necessary to design a reflector having the most suitablereflection characteristic and which efficiently utilizes ambient lightincident upon the device from every direction, and to develop atechnique for fabricating such a reflector with high accuracy and highreproducibility.

Japanese Laid-Open Patent Publication No 6-75238 discloses a reflectiveliquid crystal display device. The reflector incorporated in the liquidcrystal display device includes convex/concave portions formed of aphotosensitive resin and a film thinner than the convex/concave portionsdeposited over the convex/concave portions, thereby smoothing thesurface of the reflector including the convex/concave portions. Thereflector is used in the liquid crystal display device in combinationwith a guest-host mode (referred to as simply a “GH” mode hereinafter).

FIGS. 20G to 20L are top views each illustrating one of the fabricationsteps for a conventional reflector 106, whereas FIGS. 20A to 20F areeach cross-sectional views taken along the line A to A′ in FIGS. 20G to20L, respectively. In FIG. 20L, dashed lines represent contour lines ofthe reflector 106.

First, as shown in FIGS. 20A and 20G, a photosensitive resin isdeposited on a glass substrate 101 so as to form a photosensitive resinlayer 102 a. Then, a photomask 103 including circular regions is placedover the glass substrate 101 as shown in FIGS. 20B and 20H. Then, thesubstrate is exposed to light and developed, thus forming cylindricalprotrusions 102 b on the substrate 101 as shown in FIGS. 20C and 20I.Then, the entire substrate is subjected to a heat treatment so that theprotrusions 102 b are adequately melted and form smooth convex portions102 c as shown in FIGS. 20D and 20J. Then, a photosensitive resin isagain deposited over the entire surface of the substrate 101 includingthe smooth convex portions 102 c so as to form a photosensitive resinlayer 104 thinner than the layer 102 a, thereby obtaining a surfaceincluding smooth convex/concave portions as shown in FIGS. 20E and 20K.Finally, a thin metal film is deposited on the layer 104 so as to form areflection film 105 as shown in FIGS. 20F and 20L. A conventionalreflector 106 is thus fabricated.

As shown in FIG. 20L, the reflector 106 fabricated by the conventionalfabrication process includes a lot of flat regions. The opticalcharacteristic of the reflector 106 is such that, although nointerference occurs, a large portion of light incident upon thereflector 106 is reflected to a direction of regular reflection. Forexample, when light is incident upon the conventional reflector 106 froma direction perpendicular thereto, a large portion of the light isreflected to the direction perpendicular to the reflector 106 whichcorresponds to the direction of the regular reflection. Accordingly,there is only a very limited range of directions in which high-intensityreflected light is obtained. In other words, with such a conventionalreflector 106, it is not possible to obtain high-intensity reflectedlight in -a wide range of directions. Therefore, when such a reflectoris used in a reflective liquid crystal display device performingmulti-color display, the brightness of display would not be sufficientfor practical use.

Conventional liquid crystal display devices are produced withoutsufficient consideration for compatibility among the liquid crystaldisplay mode, the color filter, and the reflector. Therefore, there areundesirable situations such as where the display is bright but with lowcontrast; the contrast is high but with low brightness; or thebrightness and the contrast are both high but with a slow response rate,a high threshold voltage, or non-uniformity in display due to inferiororientation of the liquid crystal molecules.

In order to ensure the display quality required for practical use, theapplication of such a conventional reflective liquid crystal displaydevice is limited to a black-and-white display or, at the best, a4-color display. Thus, the growing demand for multi-color displays withthe growing variety of information has not been satisfied.

In order to realize a multi-color display which can be practically used,the compatibility among the reflector, the liquid crystal display mode,the color filter, and other factors need to be considered whileimproving the reflection characteristic of the reflector. Unlike atransmission type liquid crystal display device provided with abacklight, the reflective liquid crystal. display device greatly dependsupon ambient light. Thus, it is necessary to suitably design the opticalcharacteristic and the convex/concave structure of the reflector,appropriately select a display mode from a number of display modes tobest match the optical characteristic of the reflector, optimize variousparameters of the display mode, and appropriately design a color filter.However, it has not been possible to realize a multi-color display evenwith these factors being optimized since the reflection characteristicsof the reflector are not sufficient.

SUMMARY OF THE INVENTION

According to one aspect of this invention, a reflector includes asubstrate, a plurality of convex/concave portions formed on thesubstrate, and a thin reflective film formed over the convex/concaveportions. When light is incident upon the reflector at a first incidentangle with respect to a normal direction thereof, an intensity ofreflected light in a viewing angle range of about −45° to +45° withrespect to a regular reflection direction of the incident light is about60% or more of a reference intensity, where the reference intensity isan intensity of light which is incident upon a standard white plate at asecond incident angle with respect to a normal direction thereof and isreflected to the normal direction.

In one embodiment of the invention, each of the convex/concave portionsat least partially includes a continuous curved surface. A total area ofthe portions of the substrate whose inclination at a surface of thereflector is less than 2° accounts for about 40% or less with respect toa total area of the substrate.

In another embodiment of the invention, the convex/concave portions areformed of a photosensitive resin.

In still another embodiment of the invention, the convex/concaveportions are formed of an inorganic oxide and a photosensitive resin.

In still another embodiment of the invention, the convex/concaveportions are formed of minute particles and a photosensitive resin.

In still another embodiment of the invention, the convex/concaveportions are formed by forming a plurality of cylindrical depressions ina photosensitive resin layer formed on the substrate and heating theplurality of cylindrical depressions.

According to another aspect of this invention, a method for fabricatinga reflector including a substrate, a plurality of convex/concaveportions formed on the substrate, and a thin reflective film formed overthe convex/concave portions, includes the steps of: performing aphotolithography process and a heat-treatment process to form theconvex/concave portions for a plurality of rounds; and forming the thinreflective film over the convex/concave portions.

In one embodiment of the invention, a shape of the convex/concaveportions formed through a single round of the photolithography processis constant.

In another embodiment of the invention, a shape of the convex/concaveportions formed in one round of the photolithography process isdifferent from a shape of the convex/concave portions formed in anotherround of the photolithography process.

In still another embodiment of the invention, as a photosensitive resinused in the plurality of rounds of photolithography processes, anegative photosensitive resin is first used and a positivephotosensitive resin is subsequently used.

According to still another aspect of this invention, a method forfabricating a reflector including a substrate, convex/concave portionsformed on the substrate, and a thin reflective film formed over theconvex/concave portions, includes the steps of: performing aphotolithography process and a heat treatment to form the convex/concaveportions; and forming the thin reflective film over the convex/concaveportions. The method further includes the steps of: forming an oxide onthe substrate; and etching the oxide.

According to still another aspect of this invention, a method forfabricating a reflector including a substrate, convex/concave portionsformed on the substrate, and a thin reflective film formed over theconvex/concave portions, includes the steps of: performing aphotolithography process and a heat treatment so as to form theconvex/concave portions; and forming the thin reflective-film over theconvex/concave portions, wherein the method further comprises the stepof applying an organic insulating resin mixed with minute particles ontothe substrate.

According to still another aspect of the invention, a method forfabricating a reflector including a substrate, a plurality ofconvex/concave portions formed on the substrate, and a thin reflectivefilm formed over the convex/concave portions, includes the steps of:forming a photosensitive resin layer on the substrate; performing aphotolithography process to form a plurality of cylindrical depressionsin the photosensitive resin layer; heating the plurality of cylindricaldepressions to form the convex/concave portions on the substrate; andforming the thin reflective film over the convex/concave portions.

According to still another aspect of this invention, a reflective liquidcrystal display device includes a reflector mentioned above.

In one embodiment of the invention, the reflective liquid crystaldisplay device further includes a substrate and a liquid crystal layerinterposed between the substrate and the reflector. The liquid crystallayer includes a guest-host type liquid crystal material.

In another embodiment of the invention: a birefringence (Δn) of theliquid crystal material is about 0.15 or less; a dielectric constantanisotropy (Δ∈) of the liquid crystal material satisfies an expression:4<Δ∈<12; a twist angle of the liquid crystal material is set to bewithin a range of about 180° to 360°; and a thickness of a cellconstituted by the substrate, the reflector and the liquid crystal layeris within a range of about 3 to 10 μm.

In still another embodiment of the invention, the reflective liquidcrystal display device further includes a color filter including coloredportions of three different colors, wherein a difference in thicknessbetween adjacent colored portions is about 0.3 μm or less.

Thus, the invention described herein makes possible the advantages of(1) providing a reflector having a reflection characteristic mostsuitable for a reflective liquid crystal display device or the likeincorporating the reflector, thereby efficiently utilizing ambient lightwithout interference; (2) providing a method for fabricating the same;and (3) providing a reliable reflective liquid crystal display devicehaving excellent display quality which allows for a multi-color displayby optimizing the compatibility among the reflector, the liquid crystallayer, and the color filter.

These and other advantages of the present invention will become apparentto those skilled in the art upon reading and understanding the followingdetailed description with reference to the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1M illustrate the fabrication process of a reflectoraccording to Example 1 of the present invention.

FIG. 2A is a graph showing the inclination distribution of the surfaceof the reflector of Example 1.

FIG. 2B is a graph showing the inclination distribution of the surfaceof a reflector of Example 2.

FIG. 2C is a graph showing the inclination distribution of the surfaceof a reflector of Comparative Example 1.

FIG. 2D is a graph showing the inclination distribution of the surfaceof a reflector of Comparative Example 2.

FIG. 2E is a graph showing the inclination distribution of the surfaceof a reflector of Example 5.

FIG. 3 is a schematic diagram showing the measurement of the reflectioncharacteristic of the reflector.

FIG. 4A is a graph showing the reflection characteristic of thereflector of Example 1.

FIG. 4B is a graph showing the reflection characteristic of thereflector of Example 2.

FIG. 4C is a graph showing the reflection characteristic of thereflector of Comparative Example 1.

FIG. 4D is a graph showing the reflection characteristic of thereflector of Comparative Example 2.

FIG. 4E is a graph showing the reflection characteristic of thereflector of Example 5.

FIGS. 5A to 5I illustrate the fabrication process of a reflectoraccording to Example 2 of the present invention.

FIGS. 6A to 6J illustrate the fabrication process of a reflectoraccording to Comparative Example 2, where FIGS. 6F to 6J are top viewseach illustrating one of the fabrication steps whereas FIGS. 6A to 6 Eare cross-sectional views taken along the line A to A′ in FIGS. 6F to6J, respectively.

FIGS. 7A to 7D illustrate the fabrication process of a reflectoraccording to Example 3 of the present invention. FIGS. 8A to 8Cillustrate the fabrication process of a reflector according to Example 4of the present invention.

FIGS. 9A to 9L illustrate the fabrication process of a reflectoraccording to Example 5 of the present invention, where FIGS. 9G to 9Lare top views each illustrating one of the fabrication steps whereasFIGS. 9A to 9F are cross-sectional views taken along the line A to A′ inFIGS. 9G to 9L, respectively.

FIG. 10 is a graph illustrating the cross section of the reflectoraccording to Example 5 of the present invention.

FIG. 11 is a cross-sectional view showing the structure of a reflectiveliquid crystal display device according to Example 6 of the presentinvention.

FIG. 12 is a graph showing the relationship between a birefringence Δnand a contrast of a liquid crystal display device.

FIG. 13 is a cross-sectional view showing a color filter used inExamples 6 and 7.

FIG. 14A is a graph showing the viewing angle dependency of theintensity of the reflected light when using a reflective liquid crystaldisplay device of Example 6.

FIG. 14B is a graph showing the viewing angle dependency of theintensity of the reflected light when using a conventional reflectiveliquid crystal display device.

FIG. 15A is a graph showing the viewing angle dependency of the contrastratio of a reflective liquid crystal display device of Example 6.

FIG. 15B is a graph showing the viewing angle dependency of the contrastratio of a conventional reflective liquid crystal display device.

FIG. 16A is a diagram showing incident light obtained from a singlelight source.

FIG. 16B is a diagram showing incident light obtained from a number oflight sources.

FIG. 17A shows the range of directions to which light is reflected bythe reflector of the present invention under a single light source.

FIG. 17B shows the range of directions to which light is reflected by aconventional reflector under a single light source.

FIG. 18A shows the range of directions to which light is reflected bythe reflector of the present invention under a number of light sources.

FIG. 18B shows the range of directions to which light is reflected by aconventional reflector under a number of light sources.

FIG. 19 is a chromaticity diagram of color filters incorporated in thereflective liquid crystal display device of the present invention and acolor filter incorporated in conventional reflective liquid crystaldisplay devices.

FIGS. 20A to 20L illustrate the fabrication process of a conventionalreflector, where FIGS. 20G to 20L are top views each illustrating one ofthe fabrication steps whereas FIGS. 20A to 20F are cross-sectional viewstaken along the line A to A′ in FIGS. 20G to 20L, respectively.

FIGS. 21A to 21J illustrate the fabrication process of a reflectoraccording to Comparative Example 1, where FIGS. 21F to 21J are top viewseach illustrating one of the fabrication steps whereas FIGS. 21A to 21Eare cross-sectional views taken along the line A to A′ in FIGS. 21F to21J, respectively.

FIG. 22 is a graph showing the relationship between the surface shape ofthe reflector and the intensity of the reflected light.

FIG. 23 is a perspective view of the portion of the present inventionthat is illustrated in FIGS. 9C and 9I, depicting that the firstphotosensitive resin layer has continuity around the cylindricaldepressions.

FIG. 24 is a cross-sectional view taken along the line 24—24 in FIG. 23.

FIG. 25 is a perspective view of a prior art arrangement during aconventional reflector fabrication process, depicting shaped bumps ofnon-continuous resin material.

FIG. 26 is a cross-sectional view taken along the line 26—26 in FIG. 25.

DESCRIPTION OF THE PREFERRED EMBODIMENTS EXAMPLE 1

Hereinafter, a reflector and a method for fabricating the same accordingto Example 1 of the present invention will be described.

The method for fabricating the reflector according to Example 1 of thepresent invention will be described now with reference to FIGS. 1A to 1Millustrating the fabrication process of the reflector.

First, as shown in FIG. 1A, a photosensitive resin (e.g., “OFPR-800”:TOKYO OHKA KOGYO CO., LTD.) is spin-coated on one surface of atransparent substrate, such as a glass substrate (e.g., “7059”: CORNINGINC.) 11 at, preferably, about 500 to 3000 rpm so as to form aphotosensitive resin layer 12 a. In the present example, “OFPR-800”(TOKYO OHKA KOGYO CO., LTD.) as the photosensitive resin is applied onto“7059” (CORNING INC.) as the substrate about 1.1 mm thick for about 30seconds while spinning the glass substrate 11 at about 1000 rpm, therebyforming a photosensitive resin layer 12 a to be about 1.2 μm-thick onthe glass substrate 11. Then, the substrate is pre-baked for about 30minutes at about 100° C., after which a photomask 13 having apredetermined pattern is placed over the glass substrate 11 as shown inFIG. 1B. Then, the substrate is exposed to light and developed with adeveloping solution (e.g., 2.38% of NMD-3: TOKYO OHKA KOGYO CO., LTD.),thus forming minute cylindrical protrusions 12 b as shown in FIG. 1C.Hereinafter, the series of steps from the application of aphotosensitive resin to the exposure to light is referred to as the“photolithography process”.

The photomask 13 used at the step of FIG. 1B for forming the cylindricalprotrusions 12 b includes a plurality of minute circularlight-transmitting regions which are randomly distributed therein. Thephotomask 13 is designed so that the adjacent protrusions 12 b formed ina single photolithography process are spaced apart from one another byan interval of at least about 2 μm, whereby any protrusion 12 b is notin contact with adjacent protrusions 12 b. Further, the photomask 13 isdesigned so that the total area of all the protrusions formed throughthree rounds of photolithography processes accounts for about 80% of thetotal area of pixel regions.

After the following heat treatment process at about 120 to 250° C., theprotrusions 12 b on the substrate 11 are rounded off, thereby obtainingconvex portions 12 c, as shown in FIG. 1D, having a smooth surfacewithout any sharp edges thereon. In the present example, the heattreatment is performed at about 180° C. for about 30 minutes.Hereinafter, this step is referred to as the “heat-treatment process”.

Then, the series of steps as shown in FIGS. 1A to 1D including thephotolithography process and the heat-treatment process is repeated fora plurality of rounds (two more rounds in the present example, as shownin FIGS. 1E to 1H and 1I to 1L).

The conditions in these fabrication steps are shown in Table 1 below foreach of the three rounds of the steps.

TABLE 1 Substrate revolution during Duration of photosensitivephotosensitive Thickness of Aperture shape Aperture diameter resinapplication resin application resultant film of photomask of photomask1st round 1000 rpm 30 sec 0.5 μm Circle 20 μm 2nd round 2000 rpm 30 sec1.2 μm Circle 10 μm 3rd round 3000 rpm 30 sec 0.2 μm Circle  5 μm

According to Example 1 of the present invention, the temperature for theheat-treatment process is set to about 180° C. throughout the pluralityof rounds. However, when using a polymer with a relatively lowcross-linking property, it is preferable to set the heat-treatmenttemperature lower in a subsequent round than that in a former round forallowing the shape of the convex portions 12 c formed in the formerround to become more stable.

Then, as shown in FIG. 1M, a thin metal film 14 having alight-reflecting property (hereinafter referred to as simply the“reflection film”) is formed over the produced convex portions 12 c. Inthe present example, the reflection film 14 is formed by vacuumevaporation of Al. As well as Al, other metals (e.g., Ni, Cr, Ag) whichhave a high reflectance and can be deposited to be a thin film withoutdifficulty may be used. The reflection film 14 is preferably formed tobe about 0.01 to 1.0 μm in thickness.

A reflector 15 of Example 1 is thus obtained through the fabricationprocess described above.

When the reflector 15 is observed, the shape of the convex portion onthe surface thereof is a cone-shaped mound of gentle undulation, atleast part of which is a continuous curved surface, with the peaks ofthe convex portions being randomly distributed. Moreover, an inclinationdistribution of the surface of the reflector 15 is obtained by using aninterference microscope. The results for the reflector 15 of Example 1are shown in a graph of FIG. 2A. As shown in the graph, the total areaof regions on the surface of the reflector 15 whose inclination is 0° orgreater but less than 2° (hereinafter referred to as a “flat region”)accounts for about 17% of the total area of the pixel regions.

The reflection characteristic of the reflector 15 is measured under arealistic situation where the reflector 15 would be incorporated in anactual liquid crystal display device. FIG. 3 schematically shows how themeasurement is performed, and FIG. 4A shows the results of themeasurement for the reflector 15 of Example 1.

In FIG. 3, a UV-curable adhesive is applied onto the surface of thereflector 15 to form an adhesive layer 16 to which a glass substrate(for measurement use) 17 adheres. In order to realize realisticconditions under which a liquid crystal display device would bepractically used, a UV-curable adhesive having substantially the samerefractive index (about 1.5) as a liquid crystal layer and the glasssubstrate 17 is used to form the adhesive layer 16. It should be notedthat the interface between the adhesive layer 16 and the glass substrate17 does not affect the results of the measurement. Thus, the reflectioncharacteristic obtained through such a measurement can be considered tobe the reflection characteristic at the actual interface between thereflector 15 and a liquid crystal layer.

In the measurement, light 19 emitted from a light source 18 is madeincident upon the reflector 15 at a certain angle. Light 20 reflected atthe surface of the reflector 15 is detected by a photomultimeter 21,thereby measuring the reflection characteristic of the reflector 15. Inparticular, the intensity of the reflected light is measured whilevarying the inclination of the photomultimeter 21 (as indicated byoppositely directed arrows in FIG. 3) with respect to a direction 22 towhich the light being incident upon the reflector 15 at the certainangle and reflected by a regular reflection is directed. The inclinationof the photomultimeter 21 with respect to the regular reflectiondirection 22 is the measurement inclination 23 and corresponds to theangle from which a viewer views a reflective liquid crystal displaydevice incorporating the reflector 15 (i.e., the viewing angle of thereflector 15 ).

Throughout FIGS. 4A to 4E, the x-axis represents the measurementinclination whereas the y-axis represents the measured intensity withrespect to a reference intensity in percentage (%). Herein, themeasurement inclination is 0° when the photomultimeter 21 is in theregular reflection direction 22.

The reference intensity is obtained in the following manner. First;light is made incident upon a standard white plate (MgO). The angle ofthe light incident upon the standard white plate with respect to adirection normal to the plate is set to be substantially the same as theangle at which light is incident upon the reflector for which themeasurement is performed. Then, the light reflected by the standardwhite plate is detected by the photomaltimeter 21 which is positioned soas to receive a portion of the light reflected toward the directionnormal to the standard white plate. The intensity of the portion of thelight detected by the photomaltimeter 21 is used as the referenceintensity.

For example, a case where a measurement for a certain reflector isperformed by making light incident upon the reflector at an angle of 30°with respect to the normal direction of the reflector is described. Inthis case, the intensity of the reflected light is measured whilevarying the position of the photomaltimeter 21 with respect to theregular reflection direction 22 to which a portion of the light isdirected by the regular reflection. Thus, measured intensity isobtained. The reference intensity is obtained in a similar manner exceptfor the position of the photomaltimeter 21. More specifically, light ismade incident upon the standard white plate at an angle of 30° withrespect to the normal direction of the standard white plate. Thephotomultimeter 21 is positioned toward the normal direction of thestandard white plate, and the reference intensity is obtained as anintensity of the light reflected from the standard white plate towardthe normal direction. In another case where the measurement for thereflector is performed by making light incident upon the reflector at anangle of, for example, 50°, the incident angle at which light isincident upon the standard white plate is also set to be 50°.

In the measurement of the reflector 15 of the present example, light 19is made incident upon the reflector 15 at an angle of about 30° withrespect to the direction normal to the reflector 15. The measurementresult is presented in FIG. 4A. As shown in FIG. 4A, the reflector 15exhibits highest reflectance at a measurement inclination of about 0°.The intensity of the reflected light exceeds about 60% of the referenceintensity throughout a wide range of about −45° to +45° with respect tothe regular reflection direction. In particular, the intensity of thereflected light exceeds about 160% in a range of about −30° to +30°. Thereflector 15 of Example 1 thus provides bright display in such a widerange of viewing angle while suppressing reflection of light to theregular reflection direction (to which the conventional reflectorreflects most of light incident thereupon, thereby excessivelyincreasing the intensity of the reflected light in the direction).

The photosensitive resin material used for the reflector 15 is notlimited to the above-mentioned resin material ( “OFPR-800”: TOKYO OHKAKOGYO CO., LTD.). In fact, it is possible to use any photosensitiveresin of either a negative or positive type which can be patterned byusing at least a photolithography process. As a possible choice of thematerial, OMR-83, OMR-850, NNR-20, OFPR-2, OFPR-830, and OFPR-5000(TOKYO OHKA KOGYO CO., LTD.); TF-20, 1300-27, and 1400-27 (SHIPLEY);Photoneece (TORAY INDUSTRIES, INC.); or RW101 (SEKISUI FINE CHEMICALCO.); R101, and R633 (NIPPON KAYAKU K.K.) may be used for the reflector15 of the present invention. Herein, the pattern of the photomask 13 hasto be determined to be positive or negative depending upon whether aphotosensitive resin to be used is positive or negative.

Although in the present example, a transparent glass substrate isemployed as the substrate 11 of the reflector 15, similar effects canalso be achieved by using opaque substrates such as an Si substrate.When using an opaque substrate, there is an advantage that circuits(e.g., circuits for driving the liquid crystal display deviceincorporating the reflector) to be provided on the substrate can beeasily integrated.

Moreover, similar effects can be achieved by providing a reflector witha flat surface having more than one region having different refractiveindices. In such a case, there are advantages such as improving thepatterning of electrodes on the reflector and also improving theorientation of liquid crystal molecules.

EXAMPLE 2

Hereinafter, a reflector and a method for fabricating the same accordingto Example 2 of the present invention will be described.

The method for fabricating the reflector according to Example 2 of thepresent invention will be described now with reference to FIGS. 5A to 5Iillustrating the fabrication process of the reflector.

First, as shown in FIG. 5A a negative type photosensitive resin (e.g.,“V259PA”: NIPPON STEEL CHEMICAL Co., Ltd.) is spin-coated on one surfaceof a transparent substrate, such as a glass substrate (e.g., “7059”:CORNING INC.) 31 at, preferably, about 500 to 3000 rpm so as to form aphotosensitive resin layer 32 a to be of a desired thickness. In thepresent example, “V259PA” (NIPPON STEEL CHEMICAL Co., Ltd.) as thephotosensitive resin is applied onto “7059” (CORNING INC.) as thesubstrate about 1.1 mm thick for about 30 seconds while spinning theglass substrate 31 at about 1000 rpm, thereby forming a photosensitiveresin layer 32 a to be about 1.2 μm-thick on the glass substrate 31.Then, the substrate is pre-baked for about 30 minutes at about 100° C.,after which a photomask 33 having a predetermined pattern is placed overthe glass substrate 31 as shown in FIG. 5B. Then, the substrate isexposed to light and developed with a CaCO₃ solution (4%), thus formingminute protrusions 32 b on regions of the substrate 31 where light isblocked, as shown in FIG. 5C.

After the following heat treatment process at about 200 to 240° C., theprotrusions 32 b on the substrate 31 are rounded off, thereby obtainingconvex portions 32 c, as shown in FIG. 5D, having a smooth surfacewithout any sharp edges thereon. In the present example, the heattreatment is performed at about 220° C. for about 30 minutes.

Then, as shown in FIG. 5E, a positive type photosensitive resin isspin-coated over the substrate 31 including the convex portions 32 c at,preferably, about 500 to 3000 rpm so as to form a photosensitive resinlayer 34 a to be of a desired thickness. In the present example, “MFR”(manufactured by JAPAN SYNTHETIC RUBBER CO., LTD.) is employed as thepositive type photosensitive resin applied onto the substrate for about30 seconds while spinning the glass substrate 31 at about 2000 rpm,thereby forming a photosensitive resin layer 34 a to be about 0.5μm-thick on the glass substrate 31.

Then, the substrate is pre-baked for about 30 minutes at about 100° C.,after which a photomask 35 having a predetermined pattern is placed overthe glass substrate 31 as shown in FIG. 5F. Then, the substrate isexposed to light. The photomask 35 is designed so that no light isincident upon the convex portions 32 c of the negative typephotosensitive resin formed in the photolithography process in the firstround. Therefore, the shape of the convex portions 32 c is maintained inthe second round.

Each of the photomasks 33 and 35 used in the present example for formingthe protrusions 32 b and 34 b includes a plurality of minute circularlight-blocking regions which are randomly distributed therein. Thephotomasks 33 and 35 are designed so that the adjacent protrusions 32 band 34 b formed in a single photolithography process are spaced apartfrom one another by an interval of at least about 2 μm. Further, thephotomasks 33 and 35 are designed so that the total area of all theprotrusions formed through two rounds of photolithography processesaccounts for about 80% of the total area of pixel regions.

Then, the substrate is developed with a KOH solution (1%), thus formingthe minute protrusions 34 b on the substrate 31 as shown in FIG. 5G. Asdescribed above, by first using a negative type photosensitive resin andthen using a positive type photosensitive resin, protrusions of a stableshape can be formed. The reason is that light to be incident upon theconvex portions 32 c formed in the first round is blocked by thephotomask 35 in the exposure step in the second round.

The protrusions 34 b on the substrate 31 are rounded off by thefollowing heat treatment process at about 140 to 240° C., and are curedto be convex portions 34 c, as shown in FIG. 5H, having a smooth surfacewithout any sharp edges thereon. In the present example, the heattreatment is performed at about 180° C. for about 10 minutes.

After these steps, as shown in FIG. 5I, a reflection film 36 is formedover the produced convex portions. 32 c and 34 c on the substrate 31. Inthe present example, the reflection film 36 is formed by vacuumevaporation of Al. As well as Al, other metals (e.g., Ai, Ni, Cr, Ag)which have a high reflectance and can be deposited to be a thin filmwithout difficulty may be used to form the reflection film 36. Thereflection film 36 is preferably formed to be about 0.01 to 1.0 μm inthickness.

A reflector 37 of Example 2 is thus obtained through the fabricationprocess described above.

When the reflector 37 is observed, the shape of the convex portion onthe surface thereof is a cone-shaped mound of gentle undulation, atleast part of which is a continuous curved surface, with the peaks ofthe convex portions being randomly distributed. The diameter of theconvex portion 32 c is about 20 μm, and the diameter of the convexportion 34 c is about 10 μm. Moreover, an inclination distribution ofthe surface of the reflector 37 is obtained by using an interferencemicroscope. The results for the reflector 37 of Example 2 are shown in agraph of FIG. 2B. After the convex portions 32 c and 34 c are formedaccurately corresponding to the patterns of the photomasks 33 and 35,the total area of the flat regions (having an inclination 0° or greaterbut less than 2°) on the surface of the reflector 37 accounts for about20% of the total area of the pixel regions.

FIG. 4B shows the results of the measurement for the reflectioncharacteristic of the reflector 37 of Example 2. The measurement isperformed as in Example 1. As shown in FIG. 4B, the intensity of thereflected light exceeds about 60% of the reference intensity throughouta wide range of about −45° to +45° with respect to the regularreflection direction. Especially, the intensity of the reflected lightexceeds about 150% of the reference intensity in a range of about −30°to +30° with respect to the regular reflection direction. The reflector37 of Example 2 thus provides bright display in such a wide range of theviewing angle. In other words, reflection of light to the regularreflection direction (to which the conventional reflector reflects mostof light incident thereupon, thereby excessively increasing theintensity of the reflected light in the direction) is suppressed.

Comparative Example 1

Hereinafter, a reflector and a method for fabricating the same accordingto Comparative Example 1 will be described.

The method for fabricating the reflector according to ComparativeExample 1 will be described now with reference to FIGS. 21A to 21J,where FIGS. 21F to 21J are top views each illustrating one of thefabrication steps for a reflector 125, whereas FIGS. 21A to 21E are eachcross-sectional view taken along the line A to A′ in FIGS. 21F to 21J,respectively.

First, as shown in FIGS. 21A and 21F, a photosensitive resin isspin-coated on one surface of a transparent substrate such as a glasssubstrate 121 at, preferably, about 500 to 3000 rpm so as to form aphotosensitive resin layer 122 a to be of a desired thickness. In thepresent example, “OFPR-800” (TOKYO OHKA KOGYO CO., LTD.) as thephotosensitive resin is applied onto “7059” (CORNING INC.) as thesubstrate about 1.1 mm thick for about 30 seconds while spinning theglass substrate 121 at about 1000 rpm, thereby forming a photosensitiveresin layer 122 a to be about 1.2 μm-thick on the glass substrate 121.Then, the substrate is pre-baked for about 30 minutes at about 100° C.,after which a photomask 123 having a predetermined pattern is placedover the glass substrate 121 as shown in FIGS. 21B and 21G. Then, thesubstrate is exposed to light and developed, thus forming minutecylindrical protrusions 122 b on the substrate 121 as shown in FIGS. 21Cand 21H.

The photomask 123 used for forming the cylindrical protrusions 122 b atthe step of FIGS. 21B and 21G includes a plurality of minute circularlight-blocking regions which are randomly distributed therein. Thephotomask 123 is designed so that the adjacent protrusions 122 b formedin a single photolithography process are spaced apart from one anotherby an interval of at least about 2 μm, whereby any protrusion 122 b isnot joined with adjacent protrusions 122 b. Further, the photomask 123is designed so that the total area of all the protrusions formed throughthe photolithography process accounts for about 40% of the total area ofthe pixel regions.

The protrusions 122 b on the substrate 121 are rounded off by thefollowing heat treatment process at about 120 to 250° C., and are curedto be convex portions 122 c, as shown in FIGS. 21D and 21I, having asmooth surface without any sharp edges thereon. In the present example,the heat treatment is performed at about 180° C. for about 30 minutes.

After the steps shown in FIGS. 21E and 21J, a reflection film 124 isformed over the produced convex portions 122 c on the substrate 121. Inthe present example, the reflection film 124 is formed by vacuumevaporation of Al. As well as Al, other metals (e.g., Ai, Ni, Cr, Ag)which have a high reflectance and can be deposited to be a thin filmwithout difficulty may be used to form the reflection film 124. Thereflection film 124 is preferably formed to be about 0.01 to 1.0 μm inthickness.

A reflector 125 of Comparative Example 1 is thus obtained through thefabrication process described above.

When the reflector 125 is observed, the shape of the convex portion 122c on the surface thereof is a cone-shaped mound of gentle undulation, atleast part of which is a continuous curved surface, with the peaks ofthe convex portions 122 c being randomly distributed. Moreover, aninclination distribution of the surface of the reflector 125 is obtainedby using an interference microscope. The results for the reflector 125of Comparative Example 1 are shown in a graph of FIG. 2C. Furthermore,FIG. 4C shows the results of the measurement for the reflectioncharacteristic of the reflector 125 of Comparative Example 1. Themeasurement is performed as in Example 1.

As shown in FIG. 2C, the total area of the flat regions on the surfaceof the reflector 125 accounts for about 60% of the total area of thepixel regions.

As shown in FIG. 4C, the reflection characteristic of the reflector 125is such that the intensity of the reflected light is very high in arange of about −15° to +15° with respect to the regular reflectiondirection, whereas it rapidly decreases outside this range. Inparticular, the intensity of the reflected light in the range of about−5° to +5° with respect to the regular reflection direction is extremelyhigh as compared to that at other measurement inclinations.

The reason why a high intensity of reflected light cannot be obtained ina wide range is believed to be the excessively large area of the flatregions, whereby most of light incident thereupon is reflected by aregular reflection.

Comparative Example 2

Hereinafter, a reflector and a method for fabricating the same accordingto Comparative Example 2 will be described.

The method for fabricating the reflector according to ComparativeExample 2 will be described now with reference to FIGS. 6A to 6J, wherereference numeral 111 denotes a glass substrate; 112 a denotes aphotosensitive resin; 112 b denotes cylindrical protrusions; 112 cdenotes smooth convex portions; 114 denotes a reflection film; and 115denotes a reflector. A photomask 113 used to form the cylindricalprotrusions 112 b includes a plurality of minute circular light-blockingregions which are randomly distributed therein. In the present example,the photomask 113 is designed so that the least interval between theresultant adjacent protrusions 112 b is as small as about 0.5 μm,whereby the total area of all the protrusions becomes relatively smallwith respect to the total area of pixel regions. In particular, thephotomask 113 is designed such that the total area of all theprotrusions formed in the photolithography process accounts for about80% of the total area of pixel regions.

Other than this, the reflector 115 of Comparative Example 2 isfabricated as in Comparative Example 1 through the photolithographyprocess and the heat-treatment process.

An inclination distribution of the surface of the reflector 115 havingsuch a structure is obtained by using an interference microscope. Theresults for the reflector 115 of Comparative Example 2 are shown in agraph of FIG. 2D. As shown in the graph, the total area of the flatregions on the surface of the reflector 115 accounts for nearly 50% ofthe total area of the pixel regions.

FIG. 4D shows the results of the measurement for the reflectioncharacteristic of the reflector 115 of Comparative Example 2. Themeasurement is performed in the same manner as described in Example 1.As shown in the graph of FIG. 4D, the intensity of the reflected lightis high only in a relatively narrow range of about −15° to +15° withrespect to the regular reflection direction. In particular, theintensity of the reflected light in the range of about −5° to +5° isextremely high as compared to that at other measurement inclinations.

Although the total area of the protrusions 112 b is set at a very highproportion of about 80% with respect to the total area of the substrate,the amount of light which is reflected by the regular reflection becomesextremely large. This results from the interval between the adjacentprotrusions 112 b to be formed through a single photolithography processbeing as small as 0.5 μm. Such a small interval allows adjacentprotrusions 112 b to be joined with one another when melted, therebyconsequently increasing the total area of the flat region. Morespecifically, a number of adjacent protrusions 112 b form a massiveportion. Only the periphery portion of the massive portion forms acurvature by being deformed when heated. Therefore, the other part ofthe massive portion consequently forms a flat region.

EXAMPLE 3

Hereinafter, a reflector and a method for fabricating the same accordingto Example 3 of the present invention will be described.

The method for fabricating the reflector according to Example 3 of thepresent invention will be described now with reference to FIGS. 7A to 7Dillustrating the fabrication process of the reflector.

First, as shown in FIG. 7A an oxide is grown by sputtering on onesurface of a transparent substrate, such as a glass substrate (e.g.,“7059”: CORNING INC.) 51 so as to form an oxide film 52. As a materialfor forming the oxide film 52, inorganic oxides such as SiO₂, Al₂O₂,SiO, TiO₂, SnO₂, ITO (Indium Tin Oxide) may be used. Considering thesize of protrusions to be formed using a photosensitive resin andfurther the reflection characteristic of a reflector to be fabricated,the thickness of the oxide film 52 is preferably in the range of about0.01 to 1 μm. In the present example, SnO₂ is used to form the oxidefilm 52 on “7059” (CORNING INC.) as the substrate about 1.1 mm thick,and the oxide film 52 is grown to be about 0.1 μm in thickness.

Then, the following process is performed as the photolithographyprocess. The transparent substrate 51 including the oxide film 52 formedthereon is wet-etched by being immersed for about 10 minutes in a mixedsolution (at about 25° C.) containing hydrofluoric acid (47% solution)and nitric acid (60% solution) at a weight ratio of about 1:100. Minuteconcave/convex portions 53 are thus formed on the surface of the glasssubstrate 51 as shown in FIG. 7B.

The oxide film 52 is etched as described above, thereby forming theminute concave/convex portions 53 on the transparent substrate 51. Aphotosensitive resin is applied over the concave/convex portions 53.After a photolithography process and a heat-treatment process,concave/convex portions 54 of photosensitive resin are provided as shownin FIG. 7C. The concave/convex portions 54 may be formed by employing asimilar method as described in Example 1 or 2. As the photosensitiveresin, “MFR” (JAPAN SYNTHETIC RUBBER CO., LTD.), “OFPR-800” (TOKYO OHKAKOGYO CO., LTD.) or the like may be used.

Then, as shown in FIG. 7D, a reflection film 55 is formed over thesurfaces of the produced concave/convex portions 53 and 54 on thesubstrate 51. In the present example, the reflection film 55 is formedto be about 0.2 μm in thickness by vacuum evaporation of Al. As well asAl, other metals (e.g., Ni, Cr, Ag) which have a high reflectance andcan be deposited to be a thin film without difficulty may be used. Thereflection film 55 is preferably formed to be about 0.01 to 1.0 μm inthickness.

A reflector 56 of Example 3 is thus obtained through the fabricationprocess described above.

An inclination distribution of the surface of the reflector 56 isobtained by using an interference microscope. The surface of thereflector 56 includes the concave/convex portions 53 of the inorganicoxide and the concave/convex portions 54 of the photosensitive resin. Asdescribed above, the minute concave/convex portions 53 are formed beforeforming the concave/convex portions 54 through the photolithographyprocess and the heat-treatment process. Due to the concave/convexportions 53 formed first, the area of the flat regions on the reflector56 where the concave/convex portions 54 are not formed is reduced. As aresult, the total area of the flat regions accounts for about 40% orless with respect to the total area of the pixel regions. This reducesthe amount of light reflected by a regular reflection. When theintensity of the reflected light is actually measured in such a manneras described in Example 1, the reflector 56 exhibits the intensity ofthe reflected light exceeding about 60% of the reference intensity in awide range of about −45° to +45° with respect to the regular reflectiondirection.

In the present example, the diameter of the minute concave/convexportion 53 to be formed on the surface of the substrate 51 in advance isset to be about 2 μm. However, the diameter is not limited thereto, butcan take any value smaller than the diameter of the concave/convexportion 54 formed through the photolithography process and theheat-treatment process, and such that the adjacent concave/convexportions 53 to be formed do not overlap with one another.

EXAMPLE 4

Hereinafter, a reflector and a method for fabricating the same accordingto Example 4 of the present invention will be described.

The method for fabricating the reflector according to Example 4 of thepresent invention will be described now with reference to FIGS. 8A to 8Cillustrating the fabrication process of the reflector.

First, as shown in FIG. 8A, an organic insulating resin 63 mixed withminute particles 62 is applied onto one surface of a transparentsubstrate, such as a glass substrate. In the present example,spherically-shaped SiO₂ having a grain diameter of about 0.5 μm is usedas the particles 62, and “7059” (CORNING INC.) is used for the substrate61. As well as SiO₂, the material of the particles 62 may be glass,plastic, metal, etc. The particles may have any kind of a fixed orunfixed shape such as a spherical shape, a fibrous shape, a spindleshape, etc. The amount of the particles 62 to be mixed is preferablyabout 10% of the organic insulating resin 63. As the organic insulatingresin 63, for example, “OCD type 7” (TOKYO OHKA KOGYO CO., LTD.) may beused. Various other resins (e.g., a thermosetting resin, a photo-curingresin) may also be used for the organic insulating resin 63.

The organic insulating resin 63 mixed with the minute particles 62 isspin-coated on the surface of the substrate 61 at, preferably, about 500to 3000 rpm so as to obtain a layer having a desired thickness. In thepresent example, the organic insulating resin 63 is applied onto thesubstrate for about 30 seconds while spinning the substrate 61 at about1000 rpm. Then, a heat treatment at about 90° C. for about 3 minutes isperformed, followed by another heat treatment at about 250° C. for about60 minutes, after which the resin is cured, thus forming an organicinsulating resin layer 63 to be about 1 μm-thick on the glass substrate61.

Through the step above, a number of minute concave/convex portions ofthe particles 62 are formed on the glass substrate 61, as shown in FIG.8A.

Then, as shown in FIG. 8B, concave/convex portions 64 of aphotosensitive resin are further formed on the surface of the organicinsulating resin layer 63 by employing a similar method as described inExample 1 or 2. Subsequently, a reflection film 65 is formed as shown inFIG. 8C, thereby obtaining a reflector 66 of Example 4.

The concave/convex portions on the reflector 66 are formed by theparticles 62 and the photosensitive resin. As described above, theminute concave/convex portions 63 are formed before forming theconcave/convex portions 64 through the photolithography process and theheat-treatment process. Due to the concave/convex portions 63 formedfirst, the area of the flat regions on the reflector 66 where theconcave/convex portions 64 are not formed is reduced. As a result, thetotal area of the flat regions accounts for about 40% or less withrespect to the total area of the pixel regions. When the intensity ofthe reflected light is actually measured in such a manner as describedin Example 1, the reflector 66 exhibits the intensity of the reflectedlight exceeding about 60% of the reference intensity in a wide range ofabout −45° to +45° with respect to the regular reflection direction.

In the present example, other than mixing particles, sandblasting,polishing and the like may be employed as the method for forming theconcave/convex portions on the glass substrate 61 in advance. In thepresent example, the diameter of the minute concave/convex portion to beformed on the surface of the substrate 61 in advance is set to be about0.5 μm. However, the diameter is not limited thereto, but, dependingupon the characteristics of the resin to be employed, can take any valuesmaller than the diameter of the concave/convex portion 64 formedthrough the photolithography process, and such that the adjacentconcave/convex portions do not overlap with one another.

EXAMPLE 5

Hereinafter, a reflector and a method for fabricating the same accordingto Example 5 of the present invention will be described.

The method for fabricating the reflector according to Example 5 of thepresent invention will be described now with reference to FIGS. 9A to9L, where FIGS. 9G to 9L are top views each illustrating one of thefabrication steps for a reflector 76, whereas FIGS. 9A to 9F are eachcross-sectional view taken along the line A to A′ in FIGS. 9G to 9L,respectively.

First, the photolithography process is performed. As shown in FIGS. 9Aand 9G, a photosensitive resin is spin-coated on one surface of atransparent substrate, such as a glass substrate 71 so as to form aphotosensitive resin layer 72 a. In the present example, “OFPR-800”(TOKYO OHKA KOGYO CO., LTD.) as the photosensitive resin is applied onto“7059” (CORNING INC.) as the substrate about 1.1 mm thick so as to forma photosensitive resin layer 72 a to be about 1.2 μm-thick. Then, thesubstrate is pre-baked for about 30 minutes at about 100° C., afterwhich a photomask 73 having a predetermined pattern is placed over theglass substrate 71 as shown in FIGS. 9B and 9H. Then, the substrate isexposed to light and developed with a developing solution, (e.g., 2.38%of NMD-3: TOKYO OHKA KOGYO CO., LTD.), thus forming minute cylindricaldepressions 72 b in the photosensitive resin layer 72 a as shown inFIGS. 9C and 9I.

The photomask 73 used in the present example includes a plurality ofminute circular light-transmitting regions which are randomlydistributed therein. The photosensitive resin used in the presentexample is of a negative type. When using a negative type photosensitiveresin, however, the pattern of the photomask 73 needs to be inverted.

The cylindrical depressions 72 b in the photosensitive resin layer 72 aon the substrate 71 are rounded off by the following heat treatmentprocess at about 120 to 250° C., thereby obtaining convex/concaveportions 72 c, as shown in FIGS. 9D and 9J, having a smooth continuoussurface without any sharp edges thereon. Then, the resin is cured. Inpresent example, the heat treatment is performed at about 200° C. forabout 30 minutes.

Subsequently, a photosensitive resin is further formed on the surface ofthe convex/concave portions 72 c as shown in FIGS. 9E and 9K in order tomake the surface of the convex/concave portions 72 c even smoother. Thephotosensitive resin, which may be the same resin used for thephotosensitive resin layer 72 a, is spin-coated. The thickness of theformed photosensitive resin layer is preferably set to be about 0.3 to5.0 μm. In the present example, the thickness of the photosensitiveresin layer is set to be about 0.3 μm. Then, through another heattreatment, the photosensitive resin is deformed, thus forming a secondconvex layer 74. In the present example, the heat treatment is performedat about 200° C. for about 30 minutes. Thus, there is provided asmoother surface including convex/concave portions with reduced flatregions.

Following the above steps, as shown in FIG. 9F, a reflection film 75 isformed over the produced surface with convex/concave portions of thesubstrate 71. In FIG. 9L, -dashed lines represent contour lines of thereflection film 75. In the present example, the reflection film 75 isformed by vacuum evaporation of Al so as to be about 0.2 μm inthickness. As well as Al, any of the metals described in Example 1 maybe used. The reflection film 75 is preferably formed to be about 0.01 to1.0 μm in thickness.

A reflector 76 of Example 5 is thus obtained through the fabricationprocess described above.

Hereinafter, the characteristics of the reflector 76 of Example 5 willbe described.

FIG. 10 shows the results of a measurement of the surface shape of thereflector 76 from above by using an interference microscope. In thefigure, the x-axis represents a distance from an arbitrary point on thereflector, whereas the y-axis represents the height of theconcave/convex portions from the surface of the substrate 71. Thesurface of the reflector 76 has dimples with gentle slopes distributedat random positions as shown in cross section in FIG. 10. Moreover, theresults of measurement for the inclination distribution of the surfaceof the reflector 76 are shown in FIG. 2E. As shown in FIG. 2E, the totalarea of the flat regions on the surface of the reflector 76 accounts foronly 12% of the total area of the pixel regions.

The reflection characteristic of the reflector 76 of Example 5 ismeasured as in Example 1. The results are shown in a graph of FIG. 4E.As shown in the graph, the reflection characteristic of the reflector 76is such that the intensity of the reflected light exceeds about 60% ofthe reference intensity throughout a wide range of about −45° to +45°with respect to the regular reflection direction. In particular, in therange of about −35° to +35°0, the reflector 76 exhibited an intensity ofthe reflected light higher than the reference intensity.

As described above, in Example 5, the photosensitive resin applied overthe convex/concave portions 72 c in the second resin application ismelted when heated, thereby filling up the corners and the bottom of thedimples. Moreover, the dimples with gentle slopes formed through asingle photolithography process are distributed at random positions.Thus, the reflector 76 having desirable characteristics as thereflectors of Examples 1 to 4 is obtained.

In the present example, each of the dimples (or concave portions) formedby the photolithography process is circular (as a cross section in aplane parallel to the substrate). However, similar effect s can berealized with concave portions having polygonal cross sections.

Now, the relationship between the optical characteristics of a reflectorand the brightness of a liquid crystal display device incorporating thereflector will be described. It has been studied which opticalcharacteristic of a reflector most influences the brightness of a liquidcrystal display device incorporating the reflector (when such a liquidcrystal display device is fabricated and viewed with human eyes). As aresult of this, the viewing angle dependency of the intensity of thereflected light measured under a single light source has been found tomost influence the brightness of the liquid crystal display device asviewed under a plurality of light sources. This will be described morein detail below with reference to FIGS. 16A to 18B.

When only one light source 200 is provided as shown in FIG. 16A, light201 is incident upon a reflector 203 only from a limited range ofdirections. Accordingly, the intensity of the light 201 incident uponthe reflector 203 is high only in the limited directions. On the otherhand, when a number of light sources 200 are provided at differentpositions as shown in FIG. 16B, the light 201 is incident upon thereflector 203 from every directions. Accordingly, the intensity of theincident light 201 becomes relatively uniform in every directions ascompared to the case of the single light source.

FIG. 17A shows a reflector 204 a which is one of the reflectors ofExamples 1 to 5 exhibiting the intensity of the reflected light of about60% or more of the reference intensity in a wide range of about −45° to+45° with respect to the regular reflection direction. Under thesituation as shown in FIG. 16B, the reflector 204 a can reflect theincident light 201 toward a wide range of viewing directions(represented by a cone in FIG. 18A). It is believed that such an effectof the improved intensity of the reflected light 202 is observed sincethe light path has reversibility and, therefore, the incident light 201can be reflected toward a wide range of directions. On the other hand, areflector 204 b shown in FIG. 17B is a reflector of one of ComparativeExamples 1 and 2 which reflects a large portion of light incidentthereon by the regular reflection and therefore directs the light to alimited range of directions. Such a reflector 204 b can direct theincident light 201 only to a limited range of directions as shown inFIG. 18B and, therefore, cannot be expected to improve the brightness ofthe reflected light 202.

Brightness of display has been measured for liquid crystal displaydevices incorporating different reflectors having different reflectancesat a viewing direction inclined by about 45° from the direction of theregular reflection. The results are shown in Table 2 below. It has beenshown that a reflector having an reflectance of about 60% or higher atthe 45° inclination is needed to realize bright display.

TABLE 2 Reflectant (%) at 45° inclination from regular reflectioncomponent 30 40 50 60 70 Display brightness in a room with a pluralityof x x Δ ∘ ⊚ fluorescent lights x: Very dark Δ: Dark ∘: Bright ⊚: Verybright

Moreover, the relationship between the shape of concave/convex portionson a reflector and the optical characteristics thereof is measured. Inparticular, the total area of the flat regions on the reflector withrespect to the total area of the pixel regions and the reflectancethereof at the 45° inclination from the regular reflection direction aremeasured. The results are shown in FIG. 22. As can be seen from FIG. 22,as the area of the flat regions increases, the proportion of the amountof light reflected in the direction of the regular reflection withrespect to the amount of entire reflected light increases, therebyreducing the reflectance of the reflector in a direction inclined byabout 45° with respect to the regular reflection direction. It has beenalso shown that the total area of the flat regions has to be about 40%or less with respect to the total area of the pixel regions in order torealize bright display.

EXAMPLE 6

Hereinafter, a reflective liquid crystal display device according toExample 6 of the present invention incorporating the reflector of thepresent invention will be described.

The reflective liquid crystal display device of Example 6 incorporates areflector having a surface similar to that of the reflector of Example5, and performs display in the GH mode where no polarizer is used.

FIG. 11 shows the structure of the reflective liquid crystal displaydevice according to Example 6 of the present invention.

As shown in FIG. 11, a reflector 81 which also functions as an activematrix substrate is attached to a counter substrate 83 having a colorfilter 82 with a predetermined interval therebetween. A liquid crystallayer 84 is disposed between the reflector 81 and the counter substrate83 and sealed therein. In the present example, a GH mode liquid crystalmaterial is used for the liquid crystal layer 84. The reason is asfollows. The reflector 81 does not preserve the polarization of incidentlight very well. Accordingly, the contrast ratio of the displaydecreases when the reflector 81 is employed in a liquid crystal displaydevice which performs display in a birefringent mode where a singlepolarizer is used. The GH mode is employed in the present example forthis reason.

The structure of the reflector 81 of Example 6 will be described now.

TFTs (Thin Film Transistors) 86 are formed on a glass substrate or thelike as an insulating substrate 85. Pixel electrodes 87 are provided soas to be connected to the drain electrodes of the TFTs 86. Aphotosensitive resin layer 88 is formed so as to cover the TFTs 86 andthe pixel electrodes 87. The photosensitive resin layer 88 correspondingto the photosensitive resin layer formed on the glass substrate of thereflectors of Examples 1 to 5 has convex/concave portions formed by oneof the methods described in Examples 1 to 5. Reflective pixel electrodes89 corresponding to the reflection film in Examples 1 to 5 are formed ina matrix on the photosensitive resin layer 88, and are electricallyconnected to the pixel electrodes 87 via contact holes 90. An alignmentfilm 91 is formed so as to entirely cover the reflective pixelelectrodes 89. The surface of the reflector 81 having such a structureis similar to that of the reflector of Example 5.

On the other hand, the counter substrate 83 includes an insulatingsubstrate 85 of glass or the like. The color filter 82 including redportions, green portions and blue portions is provided on the substrate85. The thickness of the color filter 82 varies in portions of differentcolors, thereby generating difference in thickness between adjacent onesof the colored portions. A flattening layer 92 is formed on the colorfilter 82 for reducing the difference in thickness. Counter electrodes93 and an alignment film 91 are further formed in this order on theflattening layer 92.

In producing the above-described reflective liquid crystal displaydevice, conditions (primarily in respect to the characteristics of theliquid crystal layer) are optimized for the reflector 81 to be used.

First, regarding the thickness of the cell composed of the reflector 81,the counter substrate 83 and the liquid crystal layer 84, as the cell ismade thicker, the absorption of light by molecules of dye contained inthe liquid crystal layer 84 becomes higher, thereby displaying morerefined black as opposed to conventional devices. However, moreimportantly, the response rate (i.e., the rate at which the orientationof the liquid crystal and that of the dye molecules contained in theliquid crystal layer change) decreases in proportion to the square ofthe cell thickness. Therefore, considering the response rate as the mostimportant condition, the maximum cell thickness is preferably set to beabout 10 μm (more preferably, about 7 μm) so as to achieve a responserate of about 200 ms, which is necessary for practical use. On the otherhand, considering the contrast of display and for facilitating thefabrication, the minimum cell thickness is preferably set to be about 3μm (more preferably, about 4 μm). In the present example, the cell isfabricated so as to be about 5 μm in thickness. The “cell thickness” asused herein is a value obtained by subtracting the thicknesses of theresin layer and the metal layer from the thickness of the cell measuredat the contact hole, where no convex/concave portions exist.

Next, the twist angle of the liquid crystal material is preferably setto be about 180° to 360°. When employing a GH type liquid crystalmaterial, ambient light needs to be absorbed by the dye molecules whichare contained as the “guest” in the liquid crystal material as the“host” and are aligned in accordance with the orientation of the liquidcrystal molecules. For this reason, the minimum twist angle ispreferably set to be about 180°. The maximum twist angle is preferablyset to be about 360° considering the bistability of the liquid crystalmaterial. In the present example, the twist angle is set to be about240°.

Regarding the birefringence Δn of the liquid crystal material, since theincident light cannot follow the twist of the liquid crystal material insuch a range of twist angle as above, the display quality (particularlythe contrast ratio), when actually used, becomes dependent upon thebirefringence Δn. FIG. 12 shows the relationship between thebirefringence Δn of the liquid crystal material and the contrast ratio.Displayed images are generally considered “comfortable-to-view” when thecontrast ratio is about 4 or higher, and “uncomfortable-to-view” whenthe contrast ratio is less than about 3.5. Therefore, the birefringenceΔn of the liquid crystal material is preferably set to be about 0.15 orless (more preferably, about 0.10 or less). In the present example, thebirefringence Δn of the liquid crystal material is set to be about 0.09.

Moreover, in the above range of twist angle, even slight inconsistencyin the cell thickness results in stripe domains to be easily generated,thereby causing hysteresis. In such a case, gray-scale display cannot berealized. Furthermore, in the liquid crystal display device of thepresent invention, the reflector including the convex/concave portionson its surface is internally in contact with the liquid crystal layer.Therefore, the convex/concave portions on the reflector give rise to thestripe domains and thus cause hysteresis, even in a liquid crystalmaterial which is set to have a ratio d/p (d: cell thickness, p: normalpitch of the liquid crystal material) such that the stripe domains wouldnot occur on a flat reflector. In view of these disadvantages, asuitable shape of the convex/concave portions has been studied forrealizing a reflector with excellent optical characteristics and reducedgeneration of the stripe domains.

An optically active substance is added to a liquid crystal materialwhose birefringence Δn is about 0.09 so as to adjust the d/p ratio to beabout 0.58. The reflective liquid crystal display device of Example 6 isproduced using such a liquid crystal material. The liquid crystaldisplay device has a cell thickness of about 5 μm and a twist angle ofabout 240°. As the liquid crystal display device is observed through amicroscope with a voltage being applied thereto, the stripe domains aregenerated in G (green) pixels and B (blue) pixels along grooves betweenthe convex portions. This results from R (red) pixels being thicker thanthe G and B pixels as shown in a cross-sectional view of FIG. 13 showingthe color filter used in the present example. Herein, the G, B and Rpixels are pixels where green, blue and red portions, respectively, ofthe color filter are located. The greatest difference in thicknessbetween the colored portions of the color filter is observed to be about0.3 μm. It is therefore believed that the stripe domains are generatedin the G and B pixels whose cell thicknesses are less than that of the Rpixels.

A similar liquid crystal display device is produced so as to have a d/pratio of about 0.60. The stripe domains are not generated when a voltageis applied to the device. It has been shown that the d/p ratio of theliquid crystal display device needs to be about 0.6 or higher so as notto generate the stripe domains, considering the allowance for theinconsistency in the cell thickness and the allowance for the differencein thickness of the color filter. When comparing between two liquidcrystal display devices having diameters of the convex/concave portionsof about 9 μm and about 5 μm, respectively, with the d/p ratio being0.58 (pitch: about 8.6 μm) for both of the devices, the device with theconvex/concave diameter of about 5 μm has shown to generate less stripedomains than 9 μm. Therefore, it is believed that stripe domains aremore likely to be generated when the pitch of the liquid crystalmaterial used in the device is substantially equal to the diameter ofthe convex/concave portion.

Moreover, it is necessary to operate the device at a low powerconsumption level considering the voltage resistance of the drivingcircuit, the reliability of the switching element (TFT) and the liquidcrystal layer, and the portability of the device. For this reason, thedielectric constant anisotropy Δ∈ is preferably set to be about 4 to 12.As shown in Table 3, when Δ∈ is less than about 4 and the cell thicknessis about 5 μm, the threshold voltage exceeds about 3V. In such a case,there is required a driver with a high voltage resistance. However, sucha driver is an undesirable load on the switching element and the liquidcrystal layer. When Δ∈ exceeds about 12, stains and remaining images(i.e., ghost images) are likely to occur even by aging over a shortperiod of time. In the present example, Δ∈ is set to be about 7.

TABLE 3 Occurence of Liquid crystal Retention Threshold linear remainingmaterial Δε (%) voltage (V) images and stains (*) A 12.3 96.4 1.8 x B12.0 96.2 1.8 ∘ C 10.9 95.3 1.8 ∘ D 8.6 94.5 2.0 ∘ E 7.0 96.5 2.2 ⊚ F5.8 96.6 2.4 ⊚ G 5.7 96.7 2.6 ⊚ H 5.9 95.5 2.5 ⊚ I 4.8 94.8 2.7 ⊚ J 4.095.1 3.0 ⊚ K 3.5 96.0 3.4 ⊚ (*) Period of time before occurence oflinear remaining images x: Less than 200 hrs ∘: 200˜500 hrs ⊚: More than500 hrs

Thus, the reflective liquid crystal display device is produced using thereflector produced according to the present invention and employing theGH type liquid crystal material, with the parameters thereof beingoptimized. FIGS. 14A and 15A show the viewing angle characteristic ofthe reflective liquid crystal display device of Example 6; and FIGS. 14Band 15B show viewing angle characteristic of a reflective liquid crystaldisplay device incorporating a conventional reflector.

Referring to FIGS. 14A and 15A, the reflective liquid crystal displaydevice of Example 6 performs desirable display with a contrast ratio ofabout 3.5 or higher and a brightness of about 40% or higher in a widerange of about −45° to +45° with respect to the regular reflectiondirection. On the other hand, as shown in FIGS. 14B and 15B, the liquidcrystal display device incorporating the conventional reflector performsdisplay with a contrast ratio of about 3.5 or higher and a brightness ofabout 40% or higher in a limited range of about −15° to +15° withrespect to the regular reflection direction, but the brightness sharplydrops outside this range.

Moreover, when viewed by human eyes under a plurality of light sourcesas described in Example 5, the reflective liquid crystal display deviceof Example 6 not only exhibits an extended viewing angle but alsoappears brighter.

Due to the reflector of the present invention and by incorporating it ina reflective liquid crystal display device in combination with a GH typeliquid crystal material with the parameters thereof being optimized,display which is brighter than conventional devices with high contrastand no hysteresis is obtained. Thus, multi-gray-level display isrealized. Therefore, it becomes possible to combine the liquid crystaldisplay device with a color filter as will be described in Example 7. Asa result, a multi-color reflective liquid crystal display device whichcan be practically used is realized.

EXAMPLE 7

In Example 7 of the present invention, the reflective liquid crystaldisplay device of Example 6 which is adjusted to incorporate a colorfilter will be described.

FIG. 19 shows a chromaticity diagram of color filters incorporated inthe reflective liquid crystal display device of Example 7 and a colorfilter incorporated in a conventional reflective liquid crystal displaydevice. In FIG. 19, the conventional color filter (represented by Δ)only exhibits dull colors and the number of colors is limited to twobecause the amount of transmitting light must be increased. The colorfilters (represented by ◯ and ) of Example 7 exhibit three colors (red,green, and blue) which are richer than those of the conventional colorfilter.

FIG. 13 shows a cross-sectional view of the color filter used in Example7.

As shown in FIG. 13, the color filter has a maximum difference inthickness of about 0.6 μm between adjacent colored portions. When such acolor filter is attached to a counter substrate, and a liquid crystaldisplay device is produced using such a counter substrate without takingany special measures, stripe domains will be generated due to thedifference in thickness between adjacent colored portions. In order toreduce this imperfection, a flattening film is provided over the colorfilter, thereby reducing the difference in thickness between adjacentcolored portions to be about 0.3 μm. When a color filter has differencein thickness between adjacent colored portions less than 0.3 μm, thecolor filter may be used without a flattening film.

Thus, it is possible to realize a liquid crystal display device whichcan incorporate a color filter without any hysteresis. This makes itpossible to realize a display with many gray-scale levels. Accordingly,the color reproduction of the liquid crystal display device is improved.The liquid crystal display device of the present example has shown toprovide multi-color display with 256 or more colors at a low powerconsumption level, the display being excellent in brightness, contrast,response rate, reliability, etc.

As described above, according to the present invention, a reflector isprovided with a substrate, a plurality of convex/concave portions formedon the substrate, and a thin reflective film formed over theconvex/concave portions. When light is incident upon the reflector froma certain direction, an intensity of reflected light in a viewing anglerange of about −45° to +45° with respect to a regular reflectiondirection of the incident light is about 60% or more of the referenceintensity. Thus, a high intensity of reflected light can be obtained ina wider range as compared to the conventional reflector. Under ambientlight, where light is incident upon the reflector from every directions,light can be directed by the reflector to a wider viewing angle. As aresult, the total intensity of the reflected light is improved.

By forming each of the convex/concave portions at least partially toinclude a continuous curved surface, a total area of portions of thesubstrate whose inclination at a surface of the reflector is less than2° accounts for about 40% or less with respect to a total area of thesubstrate. Thus, the amount of light which is reflected by the regularreflection can be relatively reduced with respect to the entirereflected light, thereby obtaining a high intensity of reflected lightin a wide viewing angle. Moreover, under ambient light, where light isincident upon the reflector from every direction, light can be directedby the reflector to a wider viewing angle. As a result, the totalintensity of the reflected light is improved.

The convex/concave portions may be formed of a photosensitive resin.Thus, desirable convex/concave portions can be formed by aphotolithography process, not requiring a photosensitive resin foradditional patterning process. Alternatively, the convex/concaveportions may be formed of an inorganic oxide and a photosensitive resin.In this case, desirable convex/concave portions can be formed by asimple process. Alternatively, the convex/concave portions may be formedof minute particles and a photosensitive resin. In this case, desirableconvex/concave portions can be formed by a simple process.

Moreover, according to the present invention, a reflector, whichincludes a substrate, a plurality of convex/concave portions formed onthe substrate, and a thin reflective film formed over the convex/concaveportions, is fabricated by the steps of: performing a photolithographyprocess and a heat-treatment process to form the convex/concave portionsfor a plurality of rounds; and forming the thin reflective film over theconvex/concave portions. Thus, it is possible to control the density ofthe convex/concave portions on the substrate. As a result, a reflectorhaving a desirable reflection characteristic can be fabricated with highreproducibility.

By making a shape of the convex/concave portions formed through a singleround of the photolithography process constant, a reflector having adesirable reflection characteristic can be fabricated with highreproducibility based on a simple design. In addition, by making a shapeof the convex/concave portions formed in one round of thephotolithography process different from a shape of the convex/concaveportions formed in another round of the photolithography process,convex/concave portions of different shapes are formed on the substrate.Therefore, interference due to the convex/concave pattern does notoccur, and coloring of reflected light can be suppressed.

Moreover, as a photosensitive resin used in the plurality of rounds ofphotolithography processes, a negative photosensitive resin is firstused and a positive photosensitive resin is subsequently used. Thus, theshape of the convex/concave portions formed in a former step is keptunchanged.

Alternatively, a reflector, which includes a substrate, convex/concaveportions formed on the substrate, and a thin reflective film formed overthe convex/concave portions, is fabricated by the steps of: performing aphotolithography process and a heat treatment to form the convex/concaveportions; and forming the thin reflective film over the convex/concaveportions. The method further includes the steps of: forming an oxide onthe substrate; and etching the oxide. Thus, desirable convex/concaveportions can be formed by a simple process.

Alternatively, a reflector which includes a substrate, convex/concaveportions formed on the substrate, and a thin reflective film formed overthe convex/concave portions, is fabricated by the steps of: performing aphotolithography process and a heat treatment so as to form theconvex/concave portions; and forming the thin reflective film over theconvex/concave portions, wherein the method further comprises the stepof applying an organic insulating resin mixed with minute particles ontothe substrate. Thus, desirable convex/concave portions can be formed bya simple process.

Furthermore, by incorporating the reflector of the present inventioninto a reflective liquid crystal display device, it is possible torealize a display with high brightness and high contrast. In addition,the reflective liquid crystal display device may further include a layerof a guest-host type liquid crystal material interposed between thesubstrate and the reflector. In this case, it is possible to realize adisplay with higher brightness and high contrast.

In addition, a birefringence (Δn) of the liquid crystal material isabout 0.15 or less; a dielectric constant anisotropy (Δ∈) of the liquidcrystal material satisfies an expression: 4<Δ∈<12; a twist angle of theliquid crystal material is set to be within a range of about 180° to360°; and a thickness of a cell constituted by the substrate, thereflector and the liquid crystal layer is within a range of about 3 to10 μm. Thus, it is possible to realize, at a low power consumptionlevel, a reliable, bright, high-contrast, quick-response display whichcan perform display with many gray-scale levels.

Moreover, the reflective liquid crystal display device may furtherinclude a color filter including colored portions of three differentcolors, wherein a difference in thickness between adjacent coloredportions is about 0.3 μm or less. Thus, it is possible to realize amulti-color display which can be practically used.

By using the reflector of the present invention, it is possible toutilize ambient light and to obtain a high intensity of reflected lightin a wide viewing angle while suppressing the proportion of the amountof light reflected in the direction of the regular reflection withrespect to the entire reflected light. Therefore, although the intensityof ambient light is relatively uniform in every directions, the liquidcrystal display device incorporating the reflector of the presentinvention can achieve bright display under such ambient light. Moreover,in accordance with the fabrication method of the reflector of thepresent invention, the reflector as described above can be fabricatedaccurately to the design of the reflector with good reproducibility.

Furthermore, the reflective liquid crystal display device incorporatingthe reflector of the present invention allows ambient light to be moreefficiently utilized than with a conventional technique, therebyrealizing display with excellent contrast. Furthermore, by optimizingthe characteristics of the liquid crystal layer, it is possible toprovide a multi-gray-level reflective liquid crystal display devicehaving excellent characteristics in brightness, contrast, response rate,reliability, power consumption, etc., and which does not generate anyhysteresis. It is also possible, by incorporating a color filter in sucha device, to realize a multi-color reflective liquid crystal displaydevice which exhibits excellent colors.

Various other modifications will be apparent to and can be readily madeby those skilled in the art without departing from the scope and spiritof this invention. Accordingly, it is not intended that the scope of theclaims appended hereto be limited to the description as set forth-herein, but rather that the claims be broadly construed.

What is claimed is:
 1. A method for fabricating a reflector comprising asubstrate, a plurality of convex/concave portions formed on thesubstrate, and a thin reflective film formed over the convex/concaveportions, the method comprising the steps of: forming a firstphotosensitive resin layer on the substrate; performing aphotolithography process to form a plurality of cylindrical depressionsin the first photosensitive resin layer, wherein the firstphotosensitive resin layer has continuity around each of the pluralityof cylindrical depressions; heating the plurality of cylindricaldepressions to form the convex/concave portions in the firstphotosensitive resin layer on the substrate; and forming the thinreflective film over the convex/concave portions.
 2. A method forfabricating a reflector according to claim 1, further comprising a stepof forming a second photosensitive resin layer on the substrate afterthe step of heating.
 3. A method for fabricating a reflector accordingto claim 1, wherein, when light is incident upon the reflector at afirst incident angle with respect to a normal direction thereof, anintensity of the light which is reflected by the reflector toward adirection range of about −45° to +45° with respect to a regularreflection direction of the light is about 60% or more of a referenceintensity, where the reference intensity is an intensity of light whichis incident upon a standard white plate at a second incident angle withrespect to a normal direction thereof, the first incident angle and thesecond incident angle being substantially equal to each other.
 4. Amethod for fabricating a reflector according to claim 1, wherein: eachof the convex/concave portions at least partially includes a continuouscurved surface; and a total area of the portions of the substrate whoseinclination at a surface of the reflector is less than 2° accounts forabout 40% or less with respect to a total area of the substrate.
 5. Amethod for fabricating a reflector according to claim 1, wherein thedepressions are circular.
 6. A method for fabricating a reflectoraccording to claim 1, wherein the step of heating, the firstphotosensitive resin layer fills upon the corners and bottoms of theplurality of cylindrical depressions.
 7. A method for fabricating areflector comprising a substrate, a plurality of convex/concave portionsformed on the substrate, and a thin reflective film formed over theconvex/concave portions, the method comprising the steps of: forming afirst photosensitive resin layer on the substrate; performing aphotolithography process to form a plurality of cylindrical depressionsin the first photosensitive resin layer, wherein the firstphotosensitive resin layer has continuity around each of the pluralityof cylindrical depressions, and wherein each of the plurality ofcylindrical depressions is defined to extend a predetermined distancebelow the photosensitive resin layer; heating the plurality ofcylindrical depressions to form the convex/concave portions in the firstphotosensitive resin layer on the substrate; and forming the thinreflective film over the convex/concave portions.